Asthma and Smog: Does Air Pollution Cause Asthma?

Air pollution irritates this lung condition. But could smog cause asthma? (From the magazine’s archives; first published in 2006.)

The smog hanging over downtown Toronto was so thick in June 2005 that you couldn’t see the CN Tower from the mid-town restaurant where Sara La Rocque was waiting tables at an outdoor patio. That was when Sara, a 21-year-old creative writing student, quit her job to go home and strip wallpaper.

One day later, she couldn’t breathe properly. Although she had experienced asthma briefly as a child, this was different. It was like breathing through a straw. For most of the rest of the summer, as Ontario lumbered through a record summer of smog advisories, Sara stayed indoors. She was miserable.

It wasn’t until six weeks later that she could even walk around the block with the family’s golden retriever (she is not allergic to dog dander) or the Burmese mountain dog.

Sara’s pretty sure what triggered her condition, and it wasn’t the wallpaper: “My asthma is smog-induced,” she says. “I think that’s what caused it.” A few years ago, most scientists would have doubted her analysis. The common wisdom was that air pollution could only exacerbate symptoms in people already living with asthma.

When it comes to asthma, theories abound as to why it develops, starting with the hygiene hypothesis. This suggests that our urban society is too germ- and virus-free, causing the underworked immune systems of those who inherit the allergic tendency to react to proteins – such as inhaled pollen or dust mites – that should be harmless. The immune system’s over-reaction results in airway inflammation and allergic asthma attacks.

There are also new indications that antibiotics in early life and obesity may be contributing factors. But some scientists keep coming back to the relationship between asthma and air pollution, particularly to that dense layer of smog that blights our cities all too often in the summer.

Somewhere in that haze of pollution, they believe, lies an answer to the mystery of why asthma gets switched on with such frequency in the urban world.

Over the past decade, scientists have found compelling evidence that air pollution irritates the lungs and triggers attacks in those who already have asthma. Some research indicates it can also worsen asthmatic flare-ups to allergens such as pollen, dust mites or pet dander.

“Air pollution remains one of the most under-appreciated contributors to asthma exacerbation,” wrote George Thurston, an associate professor of environmental medicine at New York University School of Medicine, in a 2005 article in the Journal of Allergy and Clinical Immunology.

A classic instance: When a strike closed a steel mill in the Utah Valley for the winter during the mid-1980s, researchers found that admissions of children to hospital for asthma and pneumonia were cut in half – and they climbed right back up the following winter after the steel mill had reopened. That’s a vivid example of pollution’s effect on asthma, and there are plenty more.

California Studies Link Traffic to Onset

A group of researchers from California are now taking the relationship a step beyond the theory that air pollution triggers or worsens asthma in those who have already been diagnosed.

In a series of studies, scientists from the University of Southern California have been following thousands of California schoolchildren over several years to examine how their health is affected by the key components of smog, such as emissions from cars and trucks and ground-level ozone.

Their findings indicate that living beside a roadway greatly increases a child’s chance of developing asthma in the first place. What’s more, they’ve found that kids who take part in three or more sports breathe in high concentrations of ozone while playing, and are more likely to develop asthma than equally sporty kids playing in cleaner air.

These studies suggest that “air pollution is related to the onset and not just the exacerbation (of asthma),” says Jim Gauderman, associate professor of preventive medicine at USC’s Keck School of Medicine and one of the study co-authors. Does that mean that pollution causes asthma? “That kind of definitive statement is quite a way off,” Gauderman acknowledges in an interview.

For one thing, the pathology of how pollution might lead to the development of asthma is not clear, and scientists don’t even know which of the airborne chemicals is doing the damage.

Proving the case that pollution causes asthma will require plenty more studies – research with human populations to confirm the California findings, plus more studies of animals in the lab to show how pollution may result in asthma, Gauderman says. “Then we can talk cause.”

To even suggest that it might lead to its onset is a highly controversial proposition. Earlier studies found no evidence that air pollution increased the risk of getting asthma. What’s more, the 58 per cent increase in the prevalence of childhood asthma since 1980 happened at a time when North America’s air became cleaner, not dirtier.

As Noreen Clark, a public health professor at the University of Michigan, put it in an often cited 1999 review: “Studies illustrating the causal effect between outdoor air pollution and asthma prevalence are scant.” Clark’s skepticism is the prevailing view.

But the USC scientists are challenging that thinking. The previous researchers didn’t see the link between asthma and pollution, Gauderman says, because they relied on community-wide air monitors to assess levels of pollution. “And locally increased levels not captured by a community-wide monitor may be more important.”

Pollutants in the air can vary considerably depending on where you are in a community, and whether you’re breathing in a lot of dirty air while playing sports outdoors. By looking more closely at people who breathe in more polluted air – either because they live in a pocket of pollution or because they’re playing sports and breathing harder – the link between air pollution and asthma becomes more visible.

This website is based on journalist reporting. The information it contains is meant as general guidance and is not to be interpreted as diagnosis and treatment of individual patient conditions. For such specific medical advice, visit an allergist or pulmonologist.